1. Field of the Invention
The present invention relates to a method of hydrogenation of a thin film transistor (hereinafter abbreviated as TFT) to eliminate defects within a polycrystal silicon layer of TFT.
2. Description of Related Art
A polycrystal silicon TFT using polycrystal silicon as a channel region has been investigated widely in these years in view of introduction into an LCD (liquid crystal display) where an address circuit and a clock circuit are mounted on a substrate.
Much interest is taken in the process to form polycrystal silicon by the laser annealing method because this process enables forming of polycrystal silicon with a low temperature process on an economical substrate consisting of glass, plastics or ceramics.
Next, a structure of TFT will be explained with reference to a schematic cross-sectional view of FIG. 3. FIG. 3 illustrates a structure of a bottom gate type TFT under the condition before a passivation film is formed.
As shown in FIG. 3, a gate electrode 52 is formed on a glass substrate 51 and an oxide film 53 is formed on the surface of this gate electrode 52. A silicon nitride film 54 and a silicon oxide film 55 are then stacked thereon covering the anode oxide film 53.
On the silicon oxide film 55, a polycrystal silicon layer 56 polycrystallized by the laser crystallization method is formed. Moreover, a stopper layer 57 consisting of silicon oxide is formed on the polycrystal silicon layer 56 provided at the upper part of the gate electrode 52.
At both sides of the stopper layer 57, doped layers 58, 59 which will become the source/drain regions are formed and the polycrystal silicon layer 56 provided at the lower side of the stopper layer 57 becomes a channel region 60. The doped layers 58, 59 are formed, for example, of polycrystal silicon including an n-type impurity [phosphorus (P), arsenic (As) or antimony (Sb)].
These doped layers 58, 59 are respectively connected with the source/drain electrodes 61, 62.
The above-mentioned laser crystallization method is applied for quick crystallization of amorphous silicon layer, resulting in that crystal grain size of formed polycrystal silicon becomes small. Therefore, many grain boundaries exist. Therefore, it is required to reduce the defects of grain boundaries to manufacture a TFT having a lower leakage current.
Hydrogenation is one of such laser crystallization methods. The hydrogenation method is classified into the method where hydrogen is doped using hydrogen plasma and the method where hydrogen is diffused from amorphous silicon nitride film including hydrogen.
A method for hydrogenation of polycrystal silicon layer 56 of the TFT 50 explained with reference to FIG. 3 will then be explained hereunder.
With the plasma doping method using hydrogen plasma, hydrogen is doped, through the stopper layer 57 consisting of silicon oxide, to the polycrystal silicon layer 56 where a channel region 60 is formed. As a result, the polycrystal silicon layer 56 is hydrogenated.
As another method, after amorphous silicon nitride film including hydrogen (not illustrated) is formed, hydrogen included in this film is then diffused, through the stopper layer 57, to the polycrystal silicon layer 56 where the channel region 60 is formed. As a result, the polycrystal silicon layer 56 is hydrogenated.
Next, change of drain current--gate voltage characteristic (hereinafter described as current--voltage characteristic) of TFT 50 before or after the hydrogenation of polycrystal silicon layer 56 will be explained on the basis of the current--voltage characteristic of FIG. 4. In this figure, a drain current is plotted on the vertical axis, while a gate voltage on the horizontal axis.
As shown in the figure, the current--voltage characteristic of TFT 50 before the hydrogenation is indicated by a solid line (a). That is, increase of drain current is small with respect to the increase of a positive voltage applied. This fact suggests bad switching characteristic of TFT.
Therefore, after the hydrogenation process for about 60 minutes, the current--voltage characteristic of TFT 50 has been investigated again. As a result, this current--voltage characteristic is indicated by a broken line (b). As explained above, increase of drain current becomes larger with respect to the increase of the positive voltage applied. Namely, the ON/OFF current ratio becomes as much as six orders of magnitude, resulting in a larger gradient of the drain current--gate voltage curve, depending on decrease of a leak current because uncoupled hands of the grain boundaries are terminated by hydrogen.
For example, the effective diffusion coefficient of hydrogen in the silicon oxide is defined as 10.sup.-10 cm.sup.2 /s and the effective diffusion coefficient of hydrogen in the polycrystal silicon as 10.sup.-12 cm.sub.2 /s (1/10 of the value of the single crystal silicon). In this case, 10 seconds are required for hydrogenation of the polycrystal silicon film having a thickness of 30 nm.
The hydrogenation of the polycrystal silicon layer of TFT further requires a time longer than the abovementioned calculated time due to a delay of the diffusion of hydrogen since the polycrystal silicon layer is partly covered with the doped layer and source/drain electrode, etc. When it is taken into consideration, the hydrogenation time can be calculated as 2.5.times.10.sup.3 seconds, which almost matches the result of experiments.
When hydrogenation is further carried out, hydrogen in the polycrystal silicon layer becomes excessive, deteriorating the current--voltage characteristic.
However, the prior art methods explained above have the problems as will be explained hereunder.
1. Hydrogenation using hydrogen plasma generally reduces defects in the polycrystal silicon layer but cannot perfectly eliminate such defects because the hydrogen plasma certainly reduces defects but also generates new defects.
2. When TFT is exposed to the plasma for about 60 minutes or longer, deterioration of transistor characteristic can be observed because the channel region is irradiated continuously, while it is exposed to the plasma, to energy particles or ultraviolet ray which may generate defects through the stopper layer.
Here, characteristic change of TFT by irradiation of the ultraviolet ray will be explained hereunder with reference to the current--voltage characteristic change diagram of FIG. 5.
As shown in FIG. 5, a broken line (a) indicates the current--voltage characteristic of TFT hydrogenated by the plasma doping method using hydrogen plasma after irradiation of ultraviolet ray (mercury lamp, principal wavelength .lambda.=253.7 nm, 184.9 nm).
Moreover, a chain line (b) indicates the current--voltage characteristic after irradiation of the ultraviolet ray. As will be obvious from this figure, the current--voltage characteristic of TFT is deteriorated by irradiation of the ultraviolet rays.
A solid line (c) indicates also the current--voltage characteristic of TFT after hydrogenation by the plasma doping using hydrogen plasma. As is obvious from this characteristic, the current--voltage characteristic of TFT recovers depending on the hydrogenation process.
As explained above, FIG. 5 suggests that defects may be generated in the polycrystal silicon layer of TFT by irradiation of the ultraviolet ray.
As the hydrogen plasma used for hydrogenation, an intensive ultraviolet ray in the range from 110 nm to 162 nm is emitted. Since an energy of such ultraviolet ray is higher than the value discussed above, it can be sufficiently assumed that TFT may be damaged. Therefore, quality of TFT hydrogenated by the plasma doping using hydrogen plasma is deteriorated.
3. In the method for hydrogenating the polycrystal silicon layer using amorphous silicon nitride film including hydrogen (a-SiNx:H), since the amount of hydrogen released from the inside of the amorphous silicon nitride film is insufficient, defects cannot be eliminated perfectly by hydrogenation of the polycrystal silicon layer with the laser annealing process.